Update 2.15pm: French authorities have reopened Mont-Saint-Michel abbey to visitors after an earlier evacuation on the famed English Channel site for a visitor who apparently threatened to attack security services.

The evacuation of one of France's most-visited tourist sites came after a string of sporadic attacks around France in recent years targeting police, some of them fatal.

The site reopened this afternoon and a trickle of tourists started returning to the Mont across a promontory that connects it to the mainland.

Several hours of searching failed to turn up the suspect, and police expanded their search to neighbouring towns, according to the national gendarme service.

Details of Sunday's threat were unclear, but the gendarme service said authorities ordered the evacuation as a precaution.

During the police operation, at least three police helicopters circling over the peninsula, notably famous for being isolated by high tides.

The hilltop abbey, whose origins date to the 10th century, and surrounding sites attract more than two million visitors every year.

Police officers patrolled the steep cobblestone paths leading up to the abbey, packed with restaurants and souvenir shops and normally packed with tourists and pilgrims 365 days a year.

Thousands of people were affected by the evacuation, but the mood was calm.

Some tourists were taken out of their hotels, while others were blocked upon arrival.

"We wanted to go to Mass at the abbey. But now we can't," said Clotilde, a 23-year-old from Paris who arrived Sunday morning.

Some angry South Korean tourists shouted at an organiser trying to explain the situation.

Their group of 32 was turned back to Paris after being blocked from reaching Mont-Saint-Michel.

Tour guide Andrew Rui remained calm, however, noting that the evacuation was prompted by security reasons.

"We cannot control the situation so we accept this phenomenon," he told the AP.

The regional prosecutor's office opened an investigation into the threat, amid contradictory information about what happened.

An official with the national gendarme service said the man made the threat Sunday on one of the shuttles serving the site.

The head of the regional administration, Jean-Marc Sabathe, told broadcaster Francetvinfo that the man made the threat when he was trying to stage a street performance and got in an argument with a cafe worker.

He said the man was caught on video surveillance cameras.

"I am ordering house-by-house searches to verify if the individual is still on Mont-Saint-Michel.

"It's possible that the individual left the Mont with the flux of tourists," Mr Sabathe said.

Monks and nuns were told to stay inside the abbey during the evacuation, according to the gendarme service, and the Mont's few other residents were told to stay in their homes.

Earlier:French authorities are evacuating tourists and others from the Mont-Saint-Michel abbey and monument in western France today after a visitor apparently threatened to attack security services.

The national gendarme service said the man made the threat today on one of the shuttles serving the site, on a peninsula in the English Channel.

An official with the gendarme service said the threat was vague, but prompted authorities to order an evacuation.

Police are searching for the man who made the threat.

A shuttle service official said large crowds are being evacuated or turned away and a police helicopter is flying overhead, but the atmosphere is calm.

The site is one of the most visited in France, with several thousand tourists a day.